1. Field of the Invention
The present invention relates to a pulse width modulation circuit and related method, and more particularly, to a pulse width modulation circuit capable of linearly adjusting duty cycle with voltage and related method.
2. Description of the Prior Art
Pulse width modulation (PWM) technique is widely used for output power control, such as power management, motor control, audio amplifiers, etc. Please refer to FIG. 1, which illustrates a schematic diagram of a PWM circuit 10 in the prior art. The PWM circuit 10 includes a reference voltage generator 102, a triangular wave oscillator 104, and a comparator 106. The PWM circuit 10 is utilized for outputting a PWM signal SPWM with periodic variation of pulse width according to a result of comparing a reference voltage VR with a periodic triangular wave (or a sawtooth wave) signal VT. The prior art can control the output power by changing duty cycle of the PWM signal SPWM. Taking the U.S. patent publication document (patent publication NO. 2005/0269985) as an example, the PWM circuit 10 has a periodic triangular wave (or sawtooth wave) signal VT with fixed high and low voltage levels. As the reference voltage VR generated by the reference voltage generator 102 is linearly proportional to the input DC voltage Vin, the duty cycle of the PWM signal SPWM is linearly varied with the input DC voltage Vin. To make short of the matter, the duty cycle of the PWM signal SPWM can be linearly tunable by voltage levels of the input DC voltage Vin.
This method has some limitations and can't meet some application requirements, especially for the cases of tuning wider range of duty cycle. Modifying the design of the reference voltage generator 102 should be a solution, but it takes much more components. Another approach is to control the high and low voltage levels of the triangular wave signal VT via the input DC voltage Vin. It should be noted that the reference voltage VR is fixed for this case. In this way, the input DC voltage Vin can adjust duty cycle of the PWM signal SPWM across a wider range. However, the relation between the duty cycle of the pulse width modulation signal SPWM and the input DC voltage Vin, as shown in FIG. 2, can not achieve absolutely linearity. This nonlinear relationship should contribute some unwanted influences. For example, when a PWM signal SPWM is utilized for speed control of a fan motor, it may result in a non-linear relation between the input DC voltage Vin and the speed.